Various types of receptors belonging to the tumor necrosis factor receptor superfamily (TNFR) superfamily are involved in the regulation of cell proliferation, cell differentiation, and cell death. Also, TNFR receptors play an important role in development, of immune response, tumor necrosis, and protection against autoimmune diseases. Examples of the TNFR receptors include CD28, 4-1BB, OX40, CD40, or CD27.
Human activation-inducible tumor necrosis factor receptor (AITR) is a member of TITER superfamily, and is also called glucocorticoid-induced TNFR-related protein (GITR), tumor necrosis factor receptor superfamily member 18 (TNFRSF18), or CD357. AITR is a type I transmembrane protein of TNER superfamily. Immunoregulatory T cell (Treg cell or CD4+CD25high T cell) constitutively expresses AITR at high level, and when peripheral blood mononuclear cell (PBMC) is activated, expression of GITR is rapidly up-regulated. In particular, a signal transduction through AITR is known to inhibit a suppressive function of Treg cells, which in turn increase a resistance of CD4+ and CD8+ T cells to the suppression by Treg cells, thereby activating the CD4+ and CD8+ cells. Also, a monoclonal antibody (mAb, e.g., DTA-1) that specifically binds to GITR or a physiological ligand thereof i.e., GITRL is known to enhance antitumor immunity through various in vivo models. On the other hand, activation of GITR inhibits Treg cell which is capable of suppressing the autoreactive T cells, thereby promoting autoimmune response.
Although there has not been much information found about the biological functions of AITR and AITR ligand (AITRL), it is known that AITR is expressed at basal level in a resting cell, but its expression level rapidly increases upon activation of T cell. Also, it has been reported that AITR expression level was increased in herniated disc tissue cell isolated from lumber disc herniation patients as well as in the Treg cells from active systemic lupus erythematosus patients.
AITR binds with TNFR-associated factors (TRAFs) such as TRAF1, TRAF2 and TRAF3. When AITR interacts with its ligand, this recruits TRAF2 and ultimately activates NF-κB/NIK (Kwon, B. et al., J Biol Chem 274, 6056-6061, 1999). TRAF protein is a type of cytoplasmic adapter protein, and is involved in signal transduction for extracellular proliferation, differentiation, activation and migration. The N-terminal sites of TRAP proteins vary a lot, and all of the TRAF proteins except for TRAF1 possess a RING-finger motif at the N-terminal site thereof. A RING-finger is essential for the TRAP-mediated signal transduction. In the previous study, when the genes encoding all six types of TRAP proteins were knockout in mouse by gene targeting, various phenotypes were observed, suggesting that TRAFs have significantly various biological functions (Ha, H., Curr Protoc Immunol. Chapter 11: Unit 11.9D, 2009).
A Ca2+-dependent transcription factor known as nuclear factor of activated T cells (NFATs) regulates not only T cells but also various types of growth factors and cytokines. Also, it regulates cell-to-cell interaction molecules essential for morphogenesis, development, and other functions in various types of cells and organs. Recently, NFAT is shown to be an important factor in induction of specific genetic programs that regulate differentiation to TH lineage and effector or regulatory functions of TH cells, via the transcriptional regulation of TH lineage-specific transcription factors such as T-bet (TH1), GATA-3 (TH2), RORγt (TH17), and Foxp3 (Treg). In addition, it has been reported that NFAT family regulates transcription of various cytokines and cytokine receptors thereof (Macian, F. Nat Rev Immunol. 5, 472-484, 2005).
One of the most important factors determining the fate of CD4+ T cell is cytokine milieu during the TCR-mediated activation of naïve CD4+ cell. The major signaling pathway triggered by cytokines is an activation of the signaling transducer and activator of transcription (STAT) family proteins. The STAT proteins play an essential role in differentiation and expansion of TH cells, in particular, IFN-γ/NFAT1/STAT-1 pathway activates T-bet which is the master transcription factor specifying the TH17 lineage, while NFAT1/STAT-3 pathway activates RORγ which is the master transcription factor specifying the TH17 lineage. Also, IL-4/NFAT2/STAT-6 pathway activates GATA-3 which is the master transcription factor specifying the TH2 lineage, and NFAT1/STAT-5 pathway activates Foxp3 which is the master transcription factor specifying the Treg lineage. These complex intracellular signaling cascades play an important role in determining the differentiation of TH cell and functions thereof.
Naive CD4 T cell can differentiate into four different types of T cells such as TH1, TH2, TH17, and induced Treg cells. Each of the T cells has different function and among them, TH2 cell which produces IL-4, IL-5, IL-9, IL-10, IL-13, and IL-25 mediates immune response to parasites such as helminthes. Also, Treg cell is involved in regulation of immune response and plays an important role in maintaining self-tolerance. Increasing the number of the Treg cells or promoting a suppressive function of Treg cells are known to be important factors in treatment of autoimmune diseases and prevention of allograft rejection (Jinfang Zhu et al., Blood, 112, 1557-1569, 2008).